Field of the Invention
The present invention relates to a temperature-compensated micro-electromechanical device and to a method of temperature compensation in a micro-electromechanical device.
Description of the Related Art
As is known, the use of micro-electromechanical systems (MEMS) is increasingly widespread in several sectors of technology and has yielded encouraging results especially in the construction of inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS systems of this type usually comprise at least one mass, which is connected to a fixed body (stator) by means of springs and is movable with respect to the stator according to pre-determined degrees of freedom. The movable mass and the stator are capacitively coupled by a plurality of respective comb-fingered electrodes facing one another so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors; from which it is possible to deduce the relative displacement of the movable mass with respect to the fixed body, and hence the degree of force applied to cause the movement. On the other hand, it is also possible to apply an electrostatic force to the movable mass to set it in motion, by supplying appropriate biasing voltages.
In optimal working conditions, MEMS systems present excellent performance; in particular, MEMS inertial sensors are extremely sensitive and precise. However, a limit of currently available MEMS systems lies in the strong dependence of their response upon the temperature. In fact, also on account of their extremely small dimensions, very modest variations in temperature can produce significant strains in micro-electromechanical structures. Such strains are equivalent to relative displacements of the electrodes of the movable mass with respect to those of the stator and cause a detectable variation of the capacitive coupling between stator and movable mass. In practice, then, an offset, due to the variations in temperature, is added to the output signal of the MEMS system.
To overcome this drawback, MEMS systems are frequently incorporated in special packages, made so as to reduce the effects of thermal expansion. Alternatively, it has been proposed to use compensation circuits that electrically erase the effects of possible thermal drifts. According to one solution, for example, a nonlinear element with a temperature dependent electrical characteristic is integrated in the reading interface of the MEMS system (a diode, for example). Another technique envisages, instead, the use of a temperature sensor.
The solutions illustrated above are, however, not really satisfactory both because in any case the achievable precision is not optimal and because high costs are involved. The special packages, in fact, cannot be of a standard type and hence have very high design and fabrication costs. The compensation circuits require burdensome procedures for measuring the thermal drifts and calibrating the compensation curves and, moreover, a sufficient stability over time cannot be guaranteed.